Steel composition capable of being cold rolled

ABSTRACT

A stainless semi-ferritic steel composition suitable for forming ribbing-free cold strip said steel composition having an austenite content of between about 10 and 40 percent, at temperatures above Ac1, said steel composition comprising: Up to 0.12 % by weight carbon 13-23 % BY WEIGHT CHROMIUM 0 - 2 % BY WEIGHT MOLYBDENUM 0-0.5 % BY WEIGHT NICKEL Up to a maximum of 1.0 % by weight silicon Up to a maximum of 0.35 % by weight manganese Up to a maximum of 0.010 % by weight sulphur 0.025-0.060 % BY WEIGHT PHOSPHORUS 0.020-0.060 % BY WEIGHT ARSENIC, AT LEAST 0.050 % OF SAID ARSENIC AND SAID PHOSPHORUS BEING COMBINED ARSENIC AND PHOSPHORUS, THE BALANCE BEING IRON PLUS OTHER COMMON IMPURITIES; A PROCESS FOR OBTAINING A SEMI-FERRITIC STAINLESS STEEL WHICH IS STILL AIR-HARDENING, SAID STAINLESS STEEL HAVING A AUSTENITE CONTENT OF BETWEEN 10 AND 40 PERCENT AT TEMPERATURES ABOVE Ac1 and capable of being cold rolled into a strip of slight thickness, which strip is characterized by being substantially non-washboarding, which process comprises utilizing as the steel composition the composition set forth above, maintaining the same at a temperature of between 700* and 780*C for between 2 and 10 hours, thereafter heating the same to a hot rolling temperature just below Ac1 and rolling down to the final hot rolling thickness, said steel composition being worked either as a crude ingot from the solidus temperature or in the form of a bloom in which it is worked from the cogging temperature.

I ted States Patent Ell [ STEEL COMPOSITION CAPABLE OF BEING COLD ROLLED [75] Inventor: Maria B. Dudek, Bochum, Germany [73] Assignee: Fried. Krupp IIuttenwerke AG,

Bochum, Germany [22] Filed: Dec. 27, 1971 [21] Appl. No.: 212,794

[30] Foreign Application Priority Data Dec. 24, 1970 Germany P 20 63 666.2

[52] US. Cl. 75/126 M, 75/126 C, 75/128 W, 148/37, 148/38, 148/12, 148/135, 148/136 [51] Int. Cl. C220 39/16, C220 39/54 [58] Field of Search 75/126 C, 126 M, 75/128 W; 148/37, 38, 12, 135, 136

[56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,175,551 12/1969 Great Britain 148/12 Primary ExaminerCharles N. Lovell Att0mey-Burgess, Dinklage & Sprung [111 3,776,720 [451 Dec. 4-, 1973 Up to 0.12 by weight carbon l3-23 by weight chromium 0 2 by weight molybdenum 00.5 by weight nickel Up to a maximum of 1.0 by weight silicon Up to a maximum of 0.35 by weight manganese Up to a maximum of 0.010 by weight sulphur 0.025-0.060 by weight phosphorus 0.020-0.060 by weight arsenic,

at least 0.050 of said arsenic and said phosphorus being combined arsenic and phosphorus,

the balance being iron plus other common impurities; a process for obtaining a semi-ferritic stainless steel which is still air-hardening, said stainless steel having a austenite content of between 10 and 40 percent at temperatures above Ac and capable of being cold rolled into a strip of slight thickness, which strip is characterized by being substantially nonwashboarding, which process comprises utilizing as the steel composition the composition set forth above, maintaining the same at a temperature of between 700 and 780C for between 2 and 10 hours, thereafter heating the same to a hot rolling temperature just below Ac and rolling down to the final hot rolling thickness, said steel composition being worked either as a crude ingot from the solidus temperature or in the form of a bloom in which it is worked from the cogging temperature.

14 Claims, N0 Drawings BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This invention is directed to a novel steel composition which can be worked from a crude ingot or a bloom into a cold rolled steel having properties such that the steel does not readily provide the washboarding effect and can be cold rolled to a slight thickness and to a sheet of substantial width. More particularly, this invention is directed to a sheet of such a steel composition obtained by cooling a crude ingot or bloom at the solidus temperature or the cogging temperature, as the case may be, to between 700 and 780C and thereafter working the novel steel composition by heating the same to a hot rolling temperature just below Ac, and rolling down the same to a final hot rolling thickness.

2. DISCUSSION OF THE PRIOR ART Numerous proposals have been published which are directed to the making of hot and/or cold rolled steel strip from stainless, semi-ferritic steel containing:

Maximum 0.10 carbon 15.00 to 18.00 chromium Maximum 2.00 molybdenum Maximum 0.50 nickel plus components of common technical steel, such as silicon, manganese, phosphorus, sulphur etc., the balance being iron. Such proposals have been directed to producing it in large quantities while at the same time employing special measuresto prevent the formation of, or conditions conducive to the formation of washboarding, ribbing or roping, as it is commonly called.

One particularly effective and avantageous process for making non-washboarding hot and cold rolled strip from semi-ferritic stainless steel having the above general composition and an autenite content between and 40 percent at temperatures above Ac, involves holding the steel bloom for 2 to 10 hours at temperatures of approximately 750C while it is cooled down from the blooming temperature, and then after cooling to a lower temperature if desired, preferably to room temperature heating it up to temperature levels of 800 to 950C just under the Ac, point and hot rolling it to final hot rolled thickness.

The objective of preventing washboarding is achieved to a much greater extent than in prior art processes. The possibility of utilizing the heat of the bloom by controlled cooling offers an appreciable savings in costs in comparison to high annealing of the prior art. Also, the surface flaws produces by high annealing are prevented. Lastly, the possibility of rolling at temperatures just below Ac,, with the same or even better formability, represents a remerkable advance in the heat-treating and rolling arts.

In the present state of the art there is a growing need for wider cold-rolled strips and thinner strips. It is difficult to satisfy this need without one or more annealing processes in the course of the cold rolling, or without having to roll a hot strip down to unusually thin and difficult-to-achieve thickness. If, however, intensive cold rolling is performed with as little annealing as possible between passes, it is not easy to be sure that roping or washboarding will be prevented in all cases. The satifaction of the abovementioned needs, therefore, is a problem being faced at the present time.

SUMMARY OF THE INVENTION Broadly, this invention contemplates a stainless semiferritic steel composition capable of being rolled down to slight thickness and substantial widths, suitable for forming ribbing-free cold strip, which steel has an austenite content of between 10 and 40 percent at temperatures above Ac,, said steel having the following composition:

Up to a maximum of 0.12 by weight carbon 13-23 by weight chromium 0 2 by weight molybdenum 0 0.5 by weight nickel Up to a maximum of 1.0 by weight silicon Up to a maximum of 0.35 by weight manganese Up to a maximum of 0.010 by weight sulphur 0.025 0.060 by weight phosphorus 0.0200.060 by weight arsenic, at least 0.050

by weight of said arsenic and said phosphorus being combined arsenic andphosphorus,

the balance being iron with the usual impurities.

This invention also contemplates the method of working such a steel composition which comprises adjusting the temperature of the steel composition in either the form of a crude ingot at its solidus temperature, or in the form of a bloom at its cogging temperature, to a temperature in the range of 700 and 780 C and maintaining the same at such temperature for between 2 and 10 hours, heating said steel composition to a hot rolling temperature just below Ac, and hot-rolling said steel composition down to the final hot rolling thickness at rolling temperatures not above Ac,.

This invention also contemplates a sheet of said steel, said sheet having a thickness between 1,8 and 5 mm, preferably between 2.0 and 2.5 mm.

In accordance with this invention there are provided stainless steels which are still air-hardening having an austenite content of between 10 and 40 percent at temperatures above Ac which steels are nonwashboarding and can be cold rolled into strips of slight thickness and substantial width. The provision of a cold rolled steel which is non-washboarding and can be made into a sheet of slight thickness involves utilizing a steel having the above overall steel composition, Such a steel composition can be worked either in the form of a crude ingot, or in the form of a bloom. In the form of a crude ingot, it is worked from its solidus temperature by adjusting the temperature of the ingot to between 700 and 780C and maintaining the same at such temperature for between 2 and 10 hours. Thereafter, the steel is heated to a hot rolling temperature just below Ac, and after such a heating, the composition is rolled down to a final hot rolling thickness at rolling temperatures not above Ac,. If the steel is in the form of a bloom, it is cooled from its cogging temperature to between 700 and 780C and then processed in the same manner.

The resultant steel composition can be cold rolled without the necessity for the numerous annealing procedures utilized to provide stainless steels which are cold rolled into strips.

The steel is given a final annealing customary with cold rolled steel which serves to recrystallize'. It can be bright annealed at temperatures between 700 and 890C under an inert gas atmosphere, and such steps can be followed by pickling. The steel is remarkably free of washboarding or roping as the undesirable effeet of the stainless steel strip is referred to. Such strip can have a thickness down to 0.25 mm, and at such thickness the steel composition is characterized by only a slight washboarding at its worst. Most steel compositions, in accordance with the present invention, are characterized by no washboarding or a slight tendency of the steel to washboard.

In accordance with a preferred mode of carrying th the present invention, the steel in either a crude ingot or bloom form is cooled from its solidus or cogging temperature, as the case may be, down to an optimum temperature between. 730 and 760 C and held at such temperature for an optimum period of time between 2 and hours. Thereafter, it is cooled to a lower temperature, preferably room temperature. After being so cooled, it is heated up to a temperature between 800 and 950C, just below the Ac, point. After being maintained at such temperatures for a period of time between 4 and hours, the same is rolled down to a final hot-rolling thickness between 1.8 and 5 mm. The resultant steel composition can be cold rolled into sheets of slight thickness having improved properties with respect to washboarding, which sheets can have a substantial width.

DESCRIPTION OF PREFERRED EMBODIMENTS Surprisingly, the problem of prior art processes can be solved, pursuant to the present invention, while fundamentally fulfilling the heat treating and hot rolling conditions according to German Auslegeschrift 1,508,410, and without the use of additional measures for the prevention or control of the washboarding tendency of the hot strip. All that is required is the observation ofa certain rule in regard to the composition of the semiferritic chromium or chrome-molybdenum steel that is to be rolled.

According to the invention, in the production of nonwashboarding cold-rolled strip of slight thickness from stainless, semiferritic steel, the prior art heat treatment of the bloom or slab is performed, in which the bloom, as it is cooling down from the cogging or ingot shaping heat, is held for 2 to 10 hours at temperatures of the order of about 750C., and the next heating operation to a hot rolling temperature just below Ac, (around 800 to 950C.) is performed. Such heating to below Ac, desirably follows an intermediate cooling operation and is followed by rolling the steel to the final hotrolling thickness. The steel used in such process is a stainless, semiferritic steel which is still air-hardening at austenite content between 10 and 40 percent, and which comprises:

maximum 0.12% carbon 13-23% chromium 02% molybdenum 0-O.5% nickel maximum 1.0% silicon maximum 0.35% manganese maximum 0.010% sulfur 0.025-0.060% phosphorus 0.020-0.060% arsenic, but at least 0050 arsenic and phosphorus combined (preferably 0.060 arsenic and phosphorus combined), the balance being iron plus, in some case, other common impurities.

The terms maximum 0.l2 carbon" or up to a maximum of 0.12 by weight mean that carbon is present in such steel but not in excess of 0.12 percent by weight. This applies to other elements also.

The following alloying contents, singly or in combination are preferred:

for chromium 15-18 for molybdenum 0.01.2

for silicon max. 0.5 Furthermore, it is preferred that the manganese content be 20 to 40 times the sulphur content.

It is especially effective to adjust the ratio of the principal alloying components, manganese, sulphur, arsenic and phosphorus according to the formula:

Mn S 5 time (P As) Taking the above analysis specifications as the basis, the production of hot rolled strip will now be briefly described which is outstandingly capable of cold rolling into strip of great width and especially slight thickness, having no rooping.

If the procedure begins with a crude ingot or cast slab, the latter is cooled down from the solidus temperature to temperatures of the order of 750C and kept at those levels for at least 2 hours; then it is heated to temperatures just below Ac, and hot rolled to bloom dimensions or slab dimensions, or to the hot strip thickness.

The crude ingot or the crude slab can also be cogged in the conventional manner at high temperatures 1,250-1,000C), the bloom or slab cooled down from the cogging temperatures to the 750C level, held there for at least 2 hours, and, after reheating to temperatures just below Ac,, they can be hot-rolled to strip at temperatures not above Ac,.

If the cast ingot surface is clean, the ingot can be rolled down to the hot strip thickness in one reheating after having been held at the 750C level. The rolling temperature which at the outset wasjust below the Ac, point is sustained during the rolling because the deformation heat can easily suffice for this purpose. If the surface is less clean, after the 750C holding operation, the ingot can be cooled and the surface can be cleaned as by flame cleaning, pickling, grinding, and the like before it is reheated to a rolling temperature below Ac,.

The hot strip rolled in this manner from the steel of the above-stated composition can easily be cold-rolled without washboarding to wide, thin strip with a reduction of 60 to more than percent. Annealing between passes is not necessary. Any other measures for improving performance as regards washboarding appear to be superfluous. The final annealing which is customary with such cold rolled strip and which serves only for recrystallization, the bright annealing at 700 to 890 C under inert gas or followed by pickling, should also be performed on the cold rolled material produced in accordance with the invention.

In order to more fully illustrate the invention and the manner of practicing the same, the following examples are presented. In the table below the amount of the various components in weight percent is given.

EXAMPLES l 7 Steel compositions having the compositions set forth below in the table were prepared from crude ingot form by cooling the same down from the temperature corresponding to the solidus temperature of the ingot. They were cooled down to a temperature on the order of 750C. They were maintained at such temperatures for tions of the various steels and the amount of washboarding of the same after undergoing the reductions are reported in tabular form below.

N o. O Si Mn P S Cr M0 N1 As Hot strip cold rolled down to 0.5 mm and annealed for recrystallization. Washboarding test specimen stretched percent.

Percentage of Reduction by Cold Rolling 1 no washboarding 2 beginning of washboarding 3 very slight washboarding 4 slight washboarding 5 washboarding This shows very clearly the effect of the sulphur, manganese, phosphorus and arsenic content on the heavily cold-worked strip. Experiments made in the same manner but starting from blooms and their cogging temperatures (instead of crude ingots) showed results nearly as favorable as the results of the experiments stated from crude ingots.

It is also noted that, contrary to expectations, the increased content of the iron-associated impurities or trace elements phosphorus and arsenic had no effect on the toughness, and the resistance to corrosion was even slightly improved.

After the process discussed above, protection is also to be claimed for products consisting of the new steel composition.

The invention relates,- therefore, also to a nonwashboarding cold-rolled strip of stainless, semi-ferritic steel containing:

Maximum 0.12 carbon 13 23 chromium 0 2 molybdenum 0 0.50 nickel Maximum 1.0 silicon Maximum 0.35 manganese Maximum 0.010 sulphur 0.025 0.060 phosphorus 0.020 0.060 arsenic,

but at least 0.050 combined arsenic and.phosphorus, and preferably at least 0.060 arsenic and phosphorus combined, the balance iron, plus other common impurities. All percentages are percentages by weight.

The products made according to the invention are useful in form of the cold-rolled and bright annealed strip or sheet per se or in form of articles made of them by further cold working especially deep drawing. Strip or sheet is most farorably used for architectural purposed, parts of kitchen and sanitary equipment, such as rinsing-basins or rinsing-machines, washing-machines.

The main use of cold-worked or deep-drawn articles are those needed in motor-car industry such as hubcaps, push-rods and ornamental articles.

What is claimed is:

1. A stainless semi-ferritic steel composition suitable for forming ribbing-free cold strip said steel composition having an austenite content of between about 10 and 40 at temperatures above AC1, said steel having the following composition:

Up to 0.12 by weight carbon 13-23 by weight chromium 0 2 by weight molybdenum 0-0.5 by weight nickel Up to a maximum of 1.0 by weight silicon Up to a maximum of 0.35% by weight manganese Up to a maximum of 0.010 by weight sulphur 0.025-0.060 by weight phosphorus 0.020-0.060 by weight arsenic,

at least 0.050 of said arsenic and said phosphorus being combined arsenic and phosphorus,

the balance being iron with the usual impurities.

2. A steel composition according to claim 1, wherein chromium is present in an amount between 15 and 18 percent by weight.

3. A steel composition according to claim 1, wherein molybdenum is present in an amount between 0.9 and 1.2 weight percent.

4. A steel composition according to claim 1 containing not greater than 0.5 weight percent silicon.

5. A steel composition according to claim 1 containing chromium in an amount between 15 and 18 percent by weight, molybdenum in an amount between 0.9 and 1.2 weight percent, and not greater than 0.5 weight percent silicon.

6. A steel composition according to claim 5, wherein the amount of manganese and sulphur is less than 5 times the combined amount of phosphorus and arsenic.

7. A strip or sheet of steel having the composition of claim 1 and a thickness between 1.8 and 5 mm in the hot rolled state.

8. A rooping-free strip or sheet according to claim 7 having a thickness between 0.25 and 1.5 mm after cold rolling.

9. A method of forming a ribbing-free cold strip from the steel claimed in claim 1, comprising cooling the steel in the form of a crude ingot from solidus heat or a cogged ingot from hot working heat, holding between 700 and 780 "C for 2 to 10 hours thereby transforming the austenite content of l040 percent into ferrite and carbide, and heating to a hot rolling temperature just below Ac in the range 800950 C, rolling to the required final hot strip thickness, cooling the hot strip in 12. A method according to claim 10, wherein said steel composition is worked as a bloom and cooled from its cogging temperature down to a temperature of 750C.

13. A method according to claim 10, wherein said steel composition is rolled down to a final hot-rolling thickness between 1.8 and 5 mm.

14. A method according to claim 13, wherein said steel composition is cold-rolled down to a final coldrolling thickness between 0.25 and 1.5 mm. 

2. A steel composition according to claim 1, wherein chromium is present in an amount between 15 and 18 percent by weight.
 3. A steel composition according to claim 1, wherein molybdenum is present in an amount between 0.9 and 1.2 weight percent.
 4. A steel composition according to claim 1 containing not greater than 0.5 weight percent silicon.
 5. A steel composition according to claim 1 containing chromium in an amount between 15 and 18 percent by weight, molybdenum in an amount between 0.9 and 1.2 weight percent, and not greater than 0.5 weight percent silicon.
 6. A steel composition according to claim 5, wherein the amount of manganese and sulphur is less than 5 times the combined amount of phosphorus and arsenic.
 7. A strip or sheet of steel having the composition of claim 1 and a thickness between 1.8 and 5 mm in the hot rolled state.
 8. A rooping-free strip or sheet according to claim 7 having a thickness between 0.25 and 1.5 mm after cold rolling.
 9. A method of forming a ribbing-free cold strip from the steel claimed in claim 1, comprising cooling the steel in the form of a crude ingot from solidus heat or a cogged ingot from hot working heat, holding between 700* and 780 *C for 2 to 10 hours thereby transforming the austenite content of 10-40 percent into ferrite and carbide, and heating to a hot rolling temperature just below Ac1 in the range 800*-950 *C, rolling to the required final hot strip thickness, cooling the hot strip in air and cold rolling to the required thickness which is substantially free from longitudinal grooving.
 10. A method according to claim 9, wherein said steel composition is adjusted to an optimum temperature between 730* and 760*C and maintained at such temperature for between 2 and 5 hours and said steel composition is cooled to room temperature prior to being heated to a hot rolling temperature just below Ac1.
 11. A method according to claim 10, wherein said steel composition is worked in the form of a crude ingot cooled down from its solidus temperature to 750*C.
 12. A method according to claim 10, wherein said steel composition is worked as a bloom and cooled from its cogging temperature down to a temperature of 750*C.
 13. A method according to claim 10, wherein said steel composition is rolled down to a final hot-rolling thickness between 1.8 and 5 mm.
 14. A method according to claim 13, wherein said steel composition is cold-rolled down to a final cold-rolling thickness between 0.25 and 1.5 mm. 